At times it is clinically useful to know the area or spatial volume of a structure within the body. Examples are the cross section of the abdominal aorta and the volume of the bladder. Non-invasive bladder volume measurement techniques with ultrasound sonography have been described in the art. In principle, ultrasound scanning measures distance based on echo travel time. Early echo techniques used a single ultrasound transducer and echo presentation was recorded as echo amplitude versus depth. A method for determining bladder volume to determine residual urine volume based on distance measurement to the dorsal posterior bladder wall was described in the 1960's. The method was not adjusted to specific, filling dependent, measuring configurations.
A relation between the difference in echo travel time between echoes from the posterior an anterior bladder wall and the independently measured bladder volume was recognised. Volume measurement methods based on this observation have been described. The methods are exclusively based on bladder depth measurement. Since the bladder changes in shape when filling, a single distance measurement is not precise enough to predict the entire bladder volume. No filling dependent measurement configuration is used.
Diagnostic ultrasound is today well known for real-time cross-sectional imaging of human organs. For cross-sectional imaging the sound beam is swept electronically or mechanically through the cross section to be imaged. Echoes are presented as intensity modulated dots on a display, giving the well-known ultrasound sector scan display.
Bladder volume may be calculated based on bladder contours obtained in two orthogonal planes with a geometric assumption of bladder shape. For 3-dimensional or volumetric sonography the sound beam has to be swept through the entire organ. This further increases complexity, acquisition time of the data, and costs of the instrument. HAKENBERG ET AL: “THE ESTIMATION OF BLADDER VOLUME BY SONOCYSTOGRAPHY”, J Urol, Vol 130, pp 249-251, reported a simple method for calculating bladder volume based on measuring the diameters of the bladder from a cross sectional image taken along the midline sagittal bladder plane only. These diameters give the height and depth of the bladder at the scan plane. The bladder volume is estimated as the product of the height and depth multiplied by an empirically derived constant.
This led to a method used in the current art of performing one or more two-dimensional diagnostic ultrasound ‘B’ scans to produce images of one or more cross sections through the structure whose volume is of interest, such as the bladder, and then to make several standard reference measurements of that imaged structure which are then inserted into a formula to estimate the cross sectional area or volume as required. For the bladder, transverse and a longitudinal (sagittal) scans are recorded and the height and width of the transverse image and the depth of the longitudinal one are manually measured, then multiplied together to produce a measure of the volume. A scaling constant is usually also included within the calculation which then crudely models the volume of an oblate ellipsoid.
This crude model may have inaccuracies as high as fifty percent. The bladder varies greatly in shape. A single individual's bladder shape will vary according to the degree of filling, most closely approximating the model when significantly full. Between individuals, the shape will vary depending on a number of factors, which may change the actual bladder shape of the apparent shape as shown by an ultrasound scan. The presence or absence of a uterus will change the shape, as will the prostate. Pathology of the bladder, including haematoma, or of the surrounding organs, which may distort the bladder, will also affect the bladder shape.
An ultrasound apparatus for determining the bladder volume is shown in U.S. Pat. No. 4,926,871 to Dipankar Ganguly et al. This discloses a scan head referred to as a sparse linear array with transducers mounted at predetermined angles such that the acoustic “beams” emitted by the transducer tend to a common point. The volume is calculated according to a geometric model. An apparatus is described for automatic calculation of bladder volume from ultrasound measurements in two orthogonal planes. The device is complex, including a stepper motor for deflecting the acoustic “beams”. It requires a skilled operator to manipulate the scan head in a particular way to obtain the ultrasound measurements.
Volume measurement based on ultrasound sampling of the bladder with a hand guided transducer mounted in a pantograph has been described by Kruczkowski et al: “A non-invasive ultrasonic system to determine residual bladder volume”, IEEE Eng in Medicine & Biology Soc 10TH Ann Conf, pp 1623-1624. The sampling covers the entire bladder, follows a given pattern and is not limited to a single or two cross sections of the bladder. The acquisition procedure is time consuming and thus does not give instantaneous volume measurement results.
Apparatus exist in the prior art whereby the transducer, and thus the beam, are mechanically swept over the volume of the bladder. Such sweeping takes time, meaning that volume measurement is not available instantaneously. Further, no instantaneous feedback on optimal positioning of the apparatus with respect to the bladder is available. In an exemplary apparatus, bladder volume is measured by interrogating a three-dimensional region containing the bladder and then performing image detection on the ultrasound signals returned from the region insonated. The three dimensional scan is achieved by performing twelve planar scans rotated by mechanically sweeping a transducer through a 97 degree arc in steps of 1.9 degrees. The device is thus mechanically complex and requires complex calculations to yield a result.
Ganguly et al in U.S. Pat. No. 5,964,710 entitled “System for estimating bladder volume” disclose a method for determining bladder volume based on bladder wall contour detection from ultrasound data acquired in a plurality of planes which subdivide the bladder. In each single plane of the plurality of planes N transducers are positioned on a line to produce N ultrasound beams to measure at N positions the distance from front to back wall in the selected plan. From this the surface is derived. This procedure is repeated in the other planes as well. The volume is calculated from the weighted sum of the plurality of planes. In Ganguly's method the entire perimeter of the bladder is echographically sampled in 3 dimensions. The equipment required to undertake this sampling in a clinical context is expensive and complex.
All automated methods of determination of an organ volume will encounter the difficulty of dealing with variation in the quality of the scanned image. In a clinical setting it is not practical to always achieve optimum image quality. There may exist a wide range of brightness and contrast conditions in the scan or scans. There will be noise, and the level of this may vary widely.